For example, a gas turbine is used as an aircraft engine of, for example, a helicopter and a jet plane. The aircraft gas turbine is made up of a compressor, a combustor, and a turbine. Therefore, the compressor compresses air taken in from an air inlet to generate high-temperature and high-pressure compressed air, and the combustor supplies fuel to the compressed air to burn the fuel to generate high-temperature and high-pressure combustion gas, such that the combustion gas drives the turbine. In the case of a helicopter, a rotor is rotated by the driving force of the turbine; in the case of a jet plane, thrust is produced by the energy of exhaust gas.
The compressor, the combustor, and the turbine as components of an aircraft gas turbine are accommodated in a casing having a cylindrical shape. The casing includes, for example, a first casing to accommodate the compressor, a second casing to accommodate the combustor, and a third casing to accommodate the turbine, and the three casings are serially connected with one another. In this case, each casing is provided with a flange portion having a ring shape at an end of a case body having a cylindrical shape, and the respective flange portions are fastened by a plurality of bolts and connected with each other with the respective flange portions being closely attached to each other.
At the time of operating the aircraft gas turbine, the casing becomes high temperature because high-temperature combustion gas and exhaust gas flow into the casing. The outside of the casing is exposed to low-temperature ambient air; the inside of the casing comes into contact with the high-temperature gas. Therefore, sudden changes in temperature occur at the time of activating or stopping, thereby increasing the temperature difference between the inside and outside of the aircraft gas turbine, generating a large thermal stress, and increasing the amount of thermal deformation accordingly. Particularly, at a casing connecting portion, an increased temperature gradient from the casing body to an outer periphery of the flange portion generates a high stress exceeding a yield stress at bolt holes and the outer periphery of the flange portion. Accordingly, repetition of activating and stopping considerably decreases the service life of the flange portion because of the low-cycle fatigue acting on the flange portion.
To solve such a problem, for example, provided is a flange structure as described in Patent Literature 1 listed below. The flange structure of a pipe described in Patent Literature 1 is such that a plurality of notches depressed inward in a radial direction of the flange are formed on the outer periphery of the flange of the pipe, at intervals in a circumferential direction of the flange.